This invention relates generally to Digital Subscriber Line (DSL) technology, and more particularly to line conditioning systems and methods affording high performance broadband digital service over long local subscriber loops using ADSL (Asymmetrical DSL) technology.
Most telephone companies provide analog telephone service, often referred to as plain old telephone service (POTS), and other services to customers using a legacy infrastructure typically comprising a single twisted pair (TP) of wires for each telephone number. Each TP line is referred to as a subscriber loop or a subscriber line. POTS lines were constructed to carry a single voice signal with a 3.4 kHz bandwidth channel. DSL technology permits high-speed, high-bit rate digital transmission via lossy TP lines, but requires signal processing to overcome transmission impairments due, for example, to signal attenuation, crosstalk noise from the signals present on other lines, signal reflections, radio-frequency noise, and impulse noise. Due to the very poor high frequency performance of conventional TP lines, high-speed DSL operation (10+Mbps) is typically limited to short local loop lengths between the telephone central office (CO) and the subscriber's customer premises (CPE) of the order of 8,000-10,000 feet because of the DSL signal degradation at the higher frequencies. Signal attenuation, which is the largest component of transmission impairment, increases with frequency and line length. Accordingly, for a given transmission method the maximum achievable transmission bit rate decreases as the line length increases. The achievable data rate is also limited by other factors such as group delay, which is also a function of frequency, as well as crosstalk and noise.
Asymmetric DSL (ADSL) is a DSL technology that has a greater downstream frequency spectrum from the CO toward the CPE than the upstream frequency spectrum to the CO, and employs a much greater downstream bit rate than the upstream bit rate. This reduces near-end crosstalk, and the frequency spectrum supports simultaneous transport of duplex POTS and data on the TP. ADSL1 has a maximum signal bandwidth of the order of 1.1 MHz. ADSL2+ has a maximum signal bandwidth of the order of 2.2 MHz. Typically, the frequency spectrum between 0-30 kHz is reserved for POTS service. The spectrum between 34-125 kHz is the upstream data, and the spectrum above 164 kHz is for the downstream data.
In an effort to compete with cable service providers, some telephone service providers employ ADSL2+ technology for broadband digital data, such as Internet access, and television services (IPTV) on the same TP. Each NTSC TV channel requires approximately 4.4 Mbps when using MPEG-2 compression, and a HDTV channel requires approximately 8 Mbps. Some telephone companies are providing two channels of video-on-demand TV service as well as 3.0 Mbps of Internet broadband service, which requires a total data rate of the order of 11.8 Mbps. ADSL2+ technology can achieve this total bit rate, however, service providers can typically provide such combined video and broadband services up to local loop lengths of about 8,000 feet.
The prior application referenced above discloses an in-line amplifier for TP wire signals that enables a substantial improvement to the rate and reach of conventional ADSL technology. However, it is also desirable to obtain greater improvement and higher performance (rate and reach) to permit reliable operation of the newer DSL technologies, i.e., ADSL2+ and VDSL2, along with conventional POTS service, over greater distances than are currently available. And in doing so, it is also desirable to deploy a simple installation, inexpensive, low power consumption technology that can be powered directly from the TP lines on which it operates without disrupting the normal POTS service. It is to these ends that the present invention is directed.